Submersible turbines running on compressed gas, typically air, are known. See for example U.S. Pat. No. 272,656 to Cook, U.S. Pat. No. 271,040 to Cook, and U.S. Pat. No. 211,143 to Fogarty. Since turbines of this type run on low pressure gas, they are suitable for extracting energy from sources (e.g. low pressure gas wells) that are not otherwise useful. Moreover, such turbines are especially suited for use in environments where electric and combustion engines would be unsafe (e.g., in the presence of explosive gases). The compressed air used to run the turbine may also be used to provide necessary ventilation. However, prior art devices present several problems.
First, the submerged wheel is often provided with a housing that affords substantial clearance between the wheel and the housing at some point along the circumference. This large clearance results in the escape of air in the form of large bubbles that are released as the turbine wheel rotates. Once the air is clear of the turbine wheel, the upward motion of the air pocket is no longer available for driving the wheel. Moreover, as large amounts of water flow in to replace the air, large scale turbulence is created which extends to the surrounding water. In this manner, the efficiency of the turbine is reduced as energy is dissipated in the surrounding water. The uncontrolled influx of replacement water also causes local turbulence which produces vibrations of the turbine. The vibration both represents a loss of useful mechanical energy output and may even render the turbine output unsuitable for certain applications.
Second, typical prior art devices use a wheel submerged within a large tank of water. This results in an extremely heavy machine, possibly unsuitable for installation in existing structures. If a specially designed vessel is used, as for example a sunken well, the turbine cannot readily be moved from one location to another. Aside from the large amount of water that is required to initially put one of these turbines into operation, the large tank represents a substantial cost for material and fabrication.
A third difficulty with prior art devices relates to loading on the axle bearings due to the weight of the wheel. Excessive loading leads to frictional losses and possible ultimate failure of the bearings themselves.
A fourth difficulty with submersible turbines running on compressed air is the considerable fluid shear generated at the tips of the vanes. This represents a further loss as turbulence is set up in the surrounding water. The use of a closely-fitting housing to reduce these losses tends to localize the shear forces and thus puts an added strain on the turbine wheel.
A fifth difficulty encountered with the prior art devices involves frictional losses between the downward moving side of the turbine wheel and the water through which it moves. The turbulence set up causes vibrations which reduces the efficiency of the turbine. Prior art turbines typically use curved, back-swept vanes to cut down viscous drag and to provide more effective air entrapment. This type of vane tends to displace water outward which can cause additional turbulence.
A sixth difficulty with the prior art submersible turbines relates to the water replacement when the upwardly moving air leaves the vicinity of the rotating turbine wheel. The inflow of water tends to be relatively non-directional and therefore often acts in a direction opposite that in which the wheel is rotating. Again, the result is a reduction in the energy output of the turbine and increased vibration.